1. Field of the Invention
This invention relates to a laser source and method for producing coherent blue-green-light radiation, and more particularly relates to a miniature solid-state laser source and method for producing said radiation by second-harmonic generation of the output of a semiconductor diode laser in a nonlinear crystal that permits noncritical phase matching over a wide range of temperatures, angles and input wavelength, rendering it especially suitable for optical storage applications.
2. Description of the Prior Art
Blue-green lasers are desirable because they permit a significant enhancement of the storage density of an optical recording system. However, fundamental material fabrication problems are encountered in developing diode lasers operating at wavelengths&lt;600 nm (i.e., in the blue-green range).
Insofar as we are aware, there are only two reported examples of direct frequency doubling of diode laser radiation by second-harmonic generation (SHG). T. Taniuchi et al. reported in CLEO 87, Paper WP6, discloses generation of essentially 420 nm radiation by SHG of a GaAlAs diode laser using a LiNbO.sub.3 waveguide. However, this material does not permit phase-matched SHG at 420 nm, and the blue-green light has a curved wavefront that cannot be focussed to a diffraction-limited spot and hence is unsuitable for optical storage and many other uses. Also, LiNbO.sub.3 has a relatively low optical damage threshold due to photorefractive processes.
J. C. Baumert et al, in APPLIED OPTICS, Volume 24, page 1299 (1985) reported that frequency doubling of 860 nm GaAlAs diode lasers can be achieved with KNbO.sub.3 at room temperature; but this material has many disadvantages that severely limit its suitability for practical devices. For example, KNbO.sub.3 is difficult to grow and undergoes two phase transitions between the growth temperature and room temperature. Also the temperature and wavelength tolerances for phase matching are extremely narrow, and therefore would require a laser of very specific wavelength and also precise temperature stabilization of the SHG crystal.
Other related approaches use diode-laser pumped Nd:YAG lasers to generate 530 nm radiation by phase-matched SHG since suitable nonlinear materials are available for this process. Similarly, frequency mixing of an 808 nm diode laser and a 1.06 .mu.m Nd:YAG laser is possible in the commercially available nonlinear material KTiOPO.sub.4 (KTP), as disclosed in U.S. Pat. No. 4,791,631, issued Dec. 13, 1988, assigned to the assignee of the present invention.
There is a need for a practical nonlinear material which can be cut for noncritically phase-matched SHG of the shortest wavelength possible and combined with a semiconductor diode laser that operates at the phase-matching wavelength. Insofar as applicants are aware, coherent radiation has never heretofore been provided by SHG of a diode laser in a nonlinear crystal consisting essentially of KTP.